The present invention generally relates to electrophotography and more particularly, to a method of controlling toner concentration for an electrophotographic copying apparatus.
Conventionally, for supplying or replenishing toner in dry type electrophotographic copying apparatus, there has been employed an arrangement in which the toner is dispensed at a constant rate from a toner tank or hopper disposed on the upper portion of a developing device by gravity or through a rotating brush, roller or the like which is provided at a toner dispensing opening formed at a lower portion of the toner tank, while the amount of the toner to be supplied is externally controlled by mechanical means mainly based on the density and the like of copied images.
In the known toner dispensing device as described above, however, it is impossible to replenish the toner in proportion to an electrical charge on an electrostatic latent image which varies widely according to the density of originals to be copied. More specifically, when electrostatic latent images with low electrical charge are to be continuously developed, the amount of the toner within the developing device tends to become excessive, with the possibility that fogging and the like takes place in the copied images due to adhesion of toner onto non-image areas, or dust is produced by the excessive toner to soil the interior of the copying apparatus. On the contrary, continuous development of electrostatic latent images with high electrical charge may result in such inconveniences as reduction of image density due to deficiency of toner or undesirable adhesion of magnetizable carrier beads to the photosensitive member.
Accordingly, there has been required a toner dispensing device that will replenish toner in proportion to the density of originals, i.e. to the amount of electrical charge of the electrostatic latent image on the photosensitive member. For the above purposes, various devices have heretofore been proposed, for example, those which measure the density of the original images or copied images for controlling the amount of toner supply or those which control the toner replenishing amount by measurement of the amount of electrical charge on the surface of the photosensitive member. The proposed devices as described above which are adapted to control the toner concentration only at the toner supplying portions with the use of the rotating brush, roller and the like, however, have such drawbacks as being complex in construction, large-sized and expensive, thus failing to assure efficient and accurate control of toner concentration.
In order to overcome the disadvantages as described above, there has heretofore been proposed an arrangement, for example, in Japanese Laid Open Patent Application Tokkaisho 52-63739, in which toner is supplied to the developing device to satisfy the equation F=M+Fl=constant wherein F is the amount of toner to be supplied, Fl is the amount of toner to be recovered or collected by a toner collecting member on which a predetermined voltage opposite in polarity to the toner is impressed, and M is the amount of toner to be consumed in developing, with the toner concentration being represented by the percent of toner to carrier by weight.
Referring to FIG. 1, in the known method as described above, on the assumption that P and Q represent toner consumption and collection curves (M+Fl) respectively for no tone or entirely white originals and full tone or entirely black originals, when continuous copying of the full tone originals is effected, with toner supply in an amount of Fa and toner concentration of Da, the toner concentration remains stable at Da since the relation Fa=M+Fl is satisfied. Subsequently, upon continuous copying of the no tone originals from the above state, the toner concentration is gradually increased by the difference between the toner supplying amount Fa and curve Q so as to be finally settled at a point B, since the toner consumption and collection amount for the no tone originals is at Fb with respect to the curve Q, and at the point B, the toner concentration becomes stable at Db, since the relation Fa=M+Fl is met. From the above state, when the continuous copying is again effected for the full tone originals, the toner consumption and collection amount for the full tone originals at the concentration Db is gradually decreased by the difference between the curve P and the toner supplying amount Fa so as to be finally settled at a point A, since the toner consumption and collection amount for the full tone originals at the concentration Db is Fc from the curve P. As is seen from the foregoing, according to said known method, the toner concentration varies between the points Da and Db depending on the originals, and according to the experiment carried out by the present inventors, it has been found that when 5 weight % is taken as a reference value or central value of the variation, the variation becomes as large as 2.8 percent % at an impressed voltage of 700 V, and 4 weight % at an impressed voltage of 400 V. The large variation in the toner concentration as described above according to the originals results in such inconveniences as insufficient developing density on the low concentration side and generation of fogging on the high concentration side.
Moreover, although the triboelectrically chargeable toner is given a triboelectrical charge by stirring in the developing device, the stirring capacity of ordinary developing devices is not very large, so that insufficient triboelectrical charging takes place at the high concentration side of the concentration variation, thus giving rise to the problem that the interior of the copying apparatus is soiled by scattered toner particles. The drawbacks as described above may be eliminated to a certain extent by improving the stirring capacity, but are still extremely disadvantageous from the viewpoints of size-reduction and low cost of the copying apparatus.
Meanwhile, the variation of the toner concentration depending on the originals may be reduced by increasing the amount of toner recovery or collection with respect to the amount of developing toner so as to increase the inclinations of the curves P and Q for causing said curves P and Q to approach each other, but for the above purpose, it is required to increase the amount of toner supplied per unit time with respect to a reference value for the toner concentration. On the other hand, increasing the amount of toner supply as described above tends to cause deterioration of the toner charging, resulting in staining of the interior of the copying apparatus due to scattering of the toner.
For eliminating the disadvantages as described above, there has heretofore been proposed a new automatic toner replenishing device described in U.S. Patent Application Ser. No. 934,121 filed on August 16, 1978, now U.S. Pat. No. 4,230,070, and assigned to the same assignee as in the present application in which toner is supplied through recessed portions formed in a toner collecting roller so that part of said toner is always supplied into the developing device at a constant rate, while the remaining part of the toner is caused to contact the magnetic brush after the developing for replenishment according to the toner concentration of the developing material after the developing. While the variation of the toner concentration is reduced so that it is somewhat smaller than that in the earlier described Japanese Laid Open Patent Application Tokkaisho 52-63739, the reduction in the variation is not considered to be sufficient for substantial elimination of the disadvantages as described earlier.